A scientist carries out electrolysis of molten lead bromide using inert electrodes. Explain what happens at each electrode during this process.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.4 Electrolysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Lead bromide is an ionic compound that can be electrolysed when melted. The molten liquid acts as the electrolyte, allowing ions to move freely through it.

Model answer (4 marks)

Lead ions (Pb²⁺) are cations and migrate to the cathode, where they gain two electrons (reduction) to form lead metal:
Pb²⁺ + 2e⁻ → Pb(s)

Bromide ions (Br⁻) are anions and migrate to the anode, where they lose two electrons (oxidation) to form bromine gas:
2Br⁻ → Br₂(g) + 2e⁻

Examiner tips

Use the correct electrode names (cathode = negative, anode = positive).
Show the half‑reactions with electron transfer.
Mention the oxidation state change for each ion.

Common mistakes

Confusing cathode with anode or vice versa.
Omitting the electron transfer in the half‑reactions.

Mark scheme (4 marks)

Lead ions (Pb²⁺) are positively charged ions (cations) that move towards the cathode (negative electrode)
At the cathode, lead ions gain electrons (reduction) to form lead metal
Bromide ions (Br⁻) are negatively charged ions (anions) that move towards the anode (positive electrode)
At the anode, bromide ions lose electrons (oxidation) to form bromine gas

Key terms in this question

electrolysis · inert